1. Field of the Invention
The invention relates to a method for applying a protective silicon-containing layer to the surface of carbon or graphite bodies by plasma spraying.
2. Description of the Prior Art
Several methods have become known for protection of carbon and graphite bodies, primarily against oxidation and erosion, in which the surface of the body is coated with oxidation and erosion-resistant substances. Coating means are ceramic and compounds of refractory metals and also metals if the application temperatures of the coated body are not too high or the protection is required only for a given temperature range or a coating is needed which conducts electric current. From the group of metallic coating means, primarily silicon and alloys consisting substantially of silicon such as ferrosilicon, are technically important. These coatings means exhibit a relatively high resistance against aggressive agents and can be converted into silicon carbide entirely or partially in the generation of the protective layer itself or by a thermal post-treatment. Silicon-coated carbon and graphite has, for instance, been proposed for crucibles and other metallurgical vessels, electrodes, heat exchangers, nuclear reactors, nozzles and heat shields.
The durability of the protective layer applied to the surface of carbon and graphite bodies is determined primarily by the adhesion of the layer which is often insufficient especially in the case of repeated fast temperature changes. A separation of the layer or the formation of cracks comes about, which largely cancels the protection against oxidizing fluids. Numerous processes have been proposed for preparing protective layers on carbon and graphite which meet the requirements and consists substantially of silicon. According to U.S. Pat. No. 3,275,471, carbon and graphite bodies are immersed in a slurry of fine silicon powder with the addition of silicon carbide, and the thus coated bodies are heated to produce a protective layer which consists of a silicon matrix and silicon carbide particles dispersed in the matrix. Protective layers prepared according to this method are comparatively porous and are accordingly permeable to fluids. The behaviour or layers which are generated by deposition of silicon from the vapor phase or by flame spraying is similar. It has been proposed to eliminate the porosity of the silicon layer applied by brushing or flame spraying on graphite bodies, by local fusing, i.e. melting the silicon (British Pat. No. 866,818). With this treatment, the adhesion of the layer is improved because part of the melted material penetrates into the pores of the graphite body, and silicon carbide is formed in a transition zone. It is also known in coating smaller graphite bodies to move the body along a capillary filled with melted liquid silicon, where the melt issuing from the capillary forms a thin film on the surface of the body. In another method, carbon and graphite bodies are provided with a protective silicon layer in contact with reactive gases such as chlorosilanes at a higher temperature (German DE-OS No. 27 39 258). The protective layers prepared by this method are not free of shortcomings; in particular, the adhesion and gas tightness meet the technical requirements only in part.
Finally, it is known to provide the surface of carbon and graphite bodies with a protective layer which consists substantially of silicon by plasma spraying (German DE-OS No. 1 671 065; German Pat. No. 1 271 007). The porosity of the protective layers prepared by plasma spraying is less than the porosity of other layers except for CVD (chemical vapor deposition) layers without the diffusion of oxidizing fluids being impeded sufficiently by the layer. For this coating method, the application of several layers, the melting of the protective layer or its sealing with vitreous substances is accordingly provided as means for the necessary reduction of the permeability.